3.15 too high?
 

Ok Im not about crazy acceleration, more about good gas mileage. I can pickup a z3 3.15 lsd right now online. Just wondering if this is high? Like will my car be a wicked dog?

will kill your acceleration, but its not too tall. My 6cyl ti had a 2.93...

Stock manual diff is 3.46, automatic is 4.44 (I think...) If you do a lot of highway driving it will drop your cruising RPMs and help mpg a little. But like Xeno said, you'll lose a bit of acceleration as the trade off.

get it, you can hold insanely long second gear drifts ;)

You most likely wont see any MPG improvement.

It will drop the revs low enough to were your engine will be working harder (using more fuel) to maintain highway speed.

I know at 55mph I get better mileage in 4th gear then in fifth; however, once I reach about ~63 It changes over to being better MPG in 5th.

Ok well speaking of for now I am swapping in my standard's LSD into my automatic. That should be ok right?

Yeah, it should work, but that will be a huge drop in accel.

Its ok. I have a Ninja for going fast.

I have a Ninja for killing people. He lives in my garage.

not really sure why you want an LSD so bad. With the ratio of the LSD mated to the auto you're not going to be able to break tires loose in anything but snow/gravel anyways.

it improves traction in snow, and in New England, that's a good thing ;)

Id go for a 3.25lsd rather than a 3.15. I have a 2.5L non-vanos with 4.10lsd and it gets the same mileage as my old 3.25LSD on the highway and it revs at about 5k @75-80mph. I also have proof from a trip i took to New Hampshire, well over 40 hours of driving alone at that RPM and gas mileage was about 28-30mpg.

As far as an auto equipped ti goes, 4.10lsd is about as high as id go, for a manual, the 3.45 is the best but would prefer a 3.73 over that.

To OP: I would expect a slight increase in mileage if the numerical ratio is lowered slightly, but switching from a 4.4X:1 to 3.15 or even 3.45:1 might not help mileage. There a are a few factors going on...engine friction typically scales with the square of engine speed, so slowing the engine down does help engine efficiency. Also, at a lower rpm you will need more throttle, reducing the pumping losses across the throttle, which is a good thing. However, at too low of speeds the engine begins to fall off the range where it has good volumetric efficiency. Without having a fueling map of the engine, it's not possible to say for certain - only experiments with good repeatability can decide.